Leishmaniasis is a major global health problem with more than 12 million confirmed cases worldwide, about two million new cases being added every year and more than 350 million people at risk of being exposed to the disease. Visceral leishmaniasis (VL), the most severe form of the diseases, is often fatal if left untreated. The current options for therapies are highly limited, suffer from severe toxicities or have become ineffective due to resistance against almost all clinically-used anti-leishmanial drugs. Recent studies have led to the identification of novel tricyclic endoperoxide dimer analogs with outstanding anti-leishmanial activities against intracellular amastigotes of Leishmania donovani, the clinically-relevant parasite stages for VL. Anti-leishmanial activities of these dimers are several-fold better as compared to the current battery of clinically used anti-leishmanial drugs. Artemisinin and sodium artesunate, the parent drugs from the novel tricyclic endoperoxide dimers, have shown moderate anti-leishmanial activity, indicating selective leishmanicidal properties and a potential novel mode-of-action. One of the dimer analogs has also shown significant oral bioavailability in recent animal trials and pharmacokinetic studies in a murine model. These tricyclic endoperoxide dimers thus represent promising new leads, which can be further optimized and developed as oral treatments for drug-resistant VL. A battery of tricyclic endoperoxide dimers with various linker groups will be evaluated in vitro against intracellular amastigote stages of drug-resistant strains of L. donovani, employing newly developed parasite-rescue and transformation assay. The analogs will be simultaneously tested against a mammalian cell for cytotoxicity and determination of selectivity. The parasite- rescue and transformation assay differentiates leishmanicidal and static actions of the test compound. The lead analogs will be advanced to further evaluation against transgenic L. donovani strains in vivo for cure of VL in BALB/c mice, in vivo bioavailability/pharmacokinetics. This project will fulfill an unmet medical need of drugs for treatment of drug-resistant cases of VL, an important global health problem.